This invention relates in general to a process for the production of polycrystalline silicon and more particularly to a balanced, cyclic high pressure plasma system for the efficient production of polycrystalline silicon.
Polycrystalline silicon, used by the semiconductor industry in the production of transistors, integrated circuits, photovoltaic cells, and the like, is presently produced by the hydrogen reduction of trichlorosilane. Silicon is deposited from the silicon-bearing trichlorosilane according to the reversible reaction EQU SiHCl.sub.3 +H.sub.2 .revreaction.Si(S)+SiCl.sub.4 +SiH.sub.2 Cl.sub.2 +HCl
The reaction is carried out at an elevated temperature by mixing the gases and bringing them into contact with a heated filament deposition surface. The effluent from the reaction is a mixture of gases including unreacted SiHCl.sub.3 and H.sub.2 and the reaction products SiCl.sub.4, SiH.sub.2 Cl.sub.2 and HCl. The effluent gas mixture is separated into individual gas components by condensation, adsorption and fractional distillation techniques to recover SiHCl.sub.3 and H.sub.2 which are recycled through the reactor after adding makeup SiHCl.sub.3 and H.sub.2. The other byproducts of the reaction are not used further for silicon deposition.
Trichlorosilane is an expensive starting material. Only one-third of the reacted trichlorosilane results in the deposition of silicon; the remaining trichlorosilane is converted to low-value waste products which have little utility as silicon deposition sources. The present process is thus expensive and inefficient because of low utilization of expensive reactants.
The present process is also very energy inefficient. Large quantities of energy are expended to heat the filament to the deposition temperature. The reaction is typically carried out in a relatively uninsulated reaction chamber to insure that the walls to the reactor are maintained at a temperature well below the deposition temperature. This minimizes deposition of silicon on the reactor walls and thus enhances reactor lifetime, but results in the waste of large quantities of energy. The reaction itself is also very inefficient, partly because the filament presents a small reaction surface area.
The process of reclaiming and separating useful gases from the effluent gas mixture also consumes time and energy. Large capital investment is necessary for the reclamation equipment.
The production of polycrystalline silicon is the starting point for all silicon semiconductor devices. The cost of the polycrystalline silicon therefore influences the cost of all silicon devices. The cost is especially significant for large area devices such as silicon photovoltaic cells. Accordingly, in order to reduce the cost of semiconductor devices and especially to reduce the cost of photovoltaic devices so that they can be competitive with other energy sources it is necessary to provide efficient and cost-effective methods for the production of polycrystalline silicon.
It is therefore an object of this invention to provide a method for the production of polycrystalline silicon characterized by a high utilization of input reactants.
It is a further object of this invention to provide a cyclic method for the production of polycrystalline silicon which efficiently utilizes byproducts of the silicon-producing reaction.
It is a still further object of this invention to provide a highly energy efficient method for the production of polycrystalline silicon which utilizes a high pressure plasma reaction.
It is another object of this invention to provide a high pressure plasma method for producing polycrystalline silicon and for recycling by-products from the silicon producing reaction.